1. Field of the Invention
This invention relates in general to the optical access head for a disk drive device of the compact disc category. In particular, this invention relates to the optical grating for the optical pick-up head for high-density digital video discs (DVD) capable of operating at very high access resolution.
2. Description of Related Art
Compact disc (CD), originally developed for the optical information storage of musical data, has evolved into a diversified category of formats used for the storage of digital data in various applications. In comparison to other media of storage, the optical compact disc is a media with such inherent advantageous features as relatively large storage capacity, easy archive for long period of time, low cost, as well as integrity of contained data, among others.
Conventional magnetic-based storage media has thus been gradually replaced by compact discs whenever applications such as multimedia which require accessing large amounts of data are involved. Such applications include audio CD for high-fidelity music reproduction, video CD (VCD) that plays both video and music information, CD-Interactive (CD-I) for user interactive participation in computer entertainment and education programs, CD-recordable (CD-R) and CD-erasable (CD-E) for the mass storage of digital data for computer information archiving, and the popular CD-read-only memory (CD-ROM) for the storage of computer program and data, to name a few.
Many of these compact disc formats involve the use of computer systems and are accessible via various types of computer peripheral devices known as CD drives. As a result of rapid technical advancements in laser optics and data compression schemes, ever higher storage capacity in the compact disc media has become a highly-pursued object in this industry, aiming at ever more sophisticated applications involving large amounts of graphical, video and/or audio data.
To manufacture and utilize compact discs having high density storage capacity, high optical resolution in the read and write accesses of the compact discs are the key factor. Physical laws indicate that optical resolution is a function of two parameters, the wavelength of light used for the access of data in the disc, and the numerical aperture (NA) of the objective of the access head assembly. Essentially, both shorter wavelength and larger numerical aperture are factors for increased optical resolution.
Among these two factors in the practical optical systems built for the access of data stored in compact discs, however, larger numerical aperture require extremely high precision of the optical system constructed. In a system employing an objective with a large numerical aperture, even a slight skew of the relative position of the access head assembly with respect to the normal placement of the compact disc invites excessive imaging problems. Such imaging problems, including incorrect focusing and displaced image formation, cause difficulties in the recognition of the data received in the access operation. This represents a serious problem in the manufacture of mass-production commercial models of compact disc devices. One of the obvious solutions to this problem is to reduce the thickness of the substrate used as the physical basis of the compact disc.
Two of the widely-publicized standards intended for use in the next-generation multimedia applications are the Multi-Media CD (MMCD) and the Digital Video Disc (DVD) each supported by their respective consortium. After the initial development in the process of negotiations between the two, it appeared that a unified standard adopting the physical specifications of the DVD had been achieved.
One of the important aspects of the newly-defined DVD standard is its compatibility with the vast number of popular CD-ROMs currently in existence. In other words, the new optical peripheral devices developed for the DVD standard must be equipped with the ability to access traditional CD-ROMs in addition to its normal capability to access DVDs. To achieve this, variable focusing and tracking functionalities must be designed into the system. This way, adjustments in these functionalities can be implemented to so that the use of the system can be suitable for either the traditional CD-ROM or the new DVD media. This means a bifocal focusing system must be developed for focusing the access head of the DVD device. In addition, a tracking system with dual tracking capability must be developed.
In general, many traditional CD-ROM devices employ a three-beam tracking method, while some high-density optical disc devices employ either heterodyne tracking or differential phase detection. However, the circuitry required for access head tracking control and the subsequent data signal processing suitable for use along with the heterodyne and the differential phase detection methods is much more complicated than for the conventional three-beam method.
DVD and CD have the substrate thickness of 0.6 and 1.2 mm respectively, while the numerical aperture of their corresponding objectives are 0.6 and 0.38 respectively. Since, as mentioned above, the DVD device must be able to access both the high-density DVD and the normal CD, a bifocal design is therefore necessary. In order to further outline the background of the invention, with reference to FIGS. 1 and 2 of the accompanying drawings, description of a conventional bifocal access head is provided in the following paragraphs.
Refer to FIG. 1. A perspective view schematically shows the physical construction of a conventional bifocal pick-up head for a compact disc drive device. As is illustrated, this is a system with dual objectives 12 and 14 each compatible with the data access requirements for one compact disc format standard.
Specifically, the objective 12 having large numerical aperture and short focus length is suitable for accessing DVD, while the other lens 14 having relatively smaller numerical aperture and longer focus length is for accessing the traditional CD and CD-ROM. Use of one of the two objectives in the system is selected via some mechanical servo mechanism that switches between the two. In other words, based on the use of the type of compact disc media in the system, the suitable objective is switched into the proper position in the path of the access head light system. In such a system, due to the use of single-focus objectives, a highest possible light energy efficiency can be achieved. However, the system also suffers from the following disadvantages.
First, a precision mechanical switching servo mechanism has to be used to allow correct positioning of the selected lens into the light path. The complex and precision mechanical system adds to the complexity of the access head assembly and increases the device component's costs. Furthermore, the use of position sensors and a control system necessary for performing the lens-positioning operations adds to the production labor costs.
FIG. 2 schematically illustrates the light path system of a conventional bifocal access head assembly employing the objective HOE focusing method. In this system, the illuminating light beam is produced by the laser diode 22 which is reflected at the beam-splitter 24 and then focused by the HOE objective 26 into the data plane of the media to be accessed. Due to diffraction, the first-order light is focused at a relatively longer focal length in the data plane embedded in the thick substrate 29 for such media as the conventional CD or CD-ROM. While the zero-order light is focused at a relatively shorter focal length in the data plane embedded in the thin substrate 28 for such media as DVD. In either case, the data light beam, as reflected by the accessed media 28 or 29, is then focused again by the HOE objective 26 and forms the image on the light sensor 25 via direct passage through the beam-splitter 24. The image patterned over the surface of the sensor 25 reflects the data content of either binary 0 or 1 as accessed from the media.
This HOE bifocal scheme achieves a relatively simpler physical configuration by the integration of the HOE lens and the objective into one element. The configuration is simple enough that even the popular mechanical configuration of the conventional CD can be adopted. Imaging resolution is very good for zero-order light. However, the following disadvantages remain:
First, it is relatively difficult to implement HOE directly in an objective. Complex lens making procedures must be involved in the making of these HOE lenses. Secondly, a significant portion, typically up to about 24 percent, of the light energy is lost in the use of HOE lens.
Further comparisons between the dual objective and HOE schemes for achieving compatibility with two compact disc standards show that each has its own relative advantages. For example, based on the theory of diffraction, the HOE scheme of dual focusing has an improved imaging effect for the DVD media not found in the dual objective configuration.
Sony Corporation of Japan disclosed an imaging design in its video disc players. This was a scheme employing the use of a light filter on the back surface of the grating plate. FIGS. 3A-3D outline the concept of this design. As is clearly illustrated in the side elevational view of the grating plate 30 used in this Sony design, grating 31 was formed over the surface of the plate 30 while, over the opposite surface, a special filter coating 32 was formed. FIG. 3B schematically outlines the grating arrangement, and FIG. 3C shows the diffraction filter coating pattern arrangement. With this grating and filter coating arrangement for the Sony design, the corresponding light energy efficiency distribution pattern expressed as a function of distance along the light path is outlined in FIG. 3D. Although this design demonstrates very good characteristics, implementation of this scheme is particularly difficult, and its production involves complicated. The result is therefore very high cost.